Fuel tanks for storing liquid fuel are used, for example, for transporting or storing fuel and on vehicles for their propulsion systems. The volume of gas above the liquid fuel within the fuel tank is known as the ullage. The ullage can contain reactive components, such as oxygen and/or fuel vapours, which can be a potential fire/explosion hazard. The term “inerting” refers to the process of depleting one or more of these reactive components to reduce this potential hazard.
Various inerting systems are known in the art. For example, the potentially explosive atmosphere in the ullage can be displaced by the addition of inert gas, such as argon, nitrogen, or carbon dioxide. Particularly in the case of vehicle fuel systems, the ullage volume increases as the liquid fuel is consumed by the vehicle propulsion system and so the inert gas needs to be added while the vehicle is in motion to dilute or to eliminate ambient air ingress. Typically, the inert gas is either stored in a pressurised tank on the vehicle for use on demand, or generated on board the vehicle for immediate use.
On board generation of inert gas can have several advantages in terms of reduced weight and maintenance requirements, which are important considerations, particularly for the aircraft industry. Some known On Board Inert Gas Generation Systems (OBIGGS) for aircraft pass engine bleed air through a gas separator to produce nitrogen enriched air (NEA), which is fed to the fuel tank ullage. NEA typically has a nitrogen content of between approximately 90% to approximately 98%.
More recently a “Green” OBIGGS, or GOBIGGS system has been proposed (see, for example, WO 2007/008730) takes air and fuel vapour from the ullage and passes these over a hot catalytic bed to oxidize (combust) the fuel vapour to produce oxygen depleted air (ODA). ODA typically has an oxygen content of between approximately 0% to approximately 12%. The ODA inert gas from the GOBIGGS may be fed to the fuel tank but it contains a significant carbon dioxide and water vapour content as the products of combustion.
Water is an unavoidable contaminant in fuel but it can affect fuel system component reliability and lead to operational delays and increased maintenance activities. It is therefore desirable to decrease the water vapour content of the ODA gas before it is fed to the fuel tank ullage. However, there are many sources by which water can enter the fuel tanks. The fuel that is loaded into the tanks will contain some dissolved water, and air that enters the fuel tank via its ventilation system will contain water vapour. Accordingly, some water will inevitably be present within the fuel tanks.
The carbon dioxide within the ODA gas generated by the catalytic inerting system poses a problem because when it dissolves in the water in the fuel tanks it forms carbonic acid, which is corrosive and presents a threat to materials and structures in the fuel tank.